Current Steering Neurostimulator Device with Unidirectional Current Sources

ABSTRACT

The present disclosure provides a medical device that includes a neurostimulator. The neurostimulator includes one or more channels. Each channel includes a digitally-controlled switch coupled to a voltage source. The switch is in one of an “on” state and an “off” state in response to a first control signal. Each channel also includes a digitally-controlled current sink coupled to the switch. The current sink is coupled between the switch and the voltage source. The current sink draws a variable amount of electrical current in response to a second control signal. Each channel further includes a conductor coupled to the switch and the current sink. The conductor is configured to be coupled to an electrode that is operable to deliver the electrical current drawn by the current sink to a target tissue area.

BACKGROUND

As medical device technologies continue to evolve, neurostimulatordevices have gained much popularity in the medical field.Neurostimulator devices are electrically-powered devices (e.g.,battery-powered) that are designed to deliver electrical stimulation toa patient. Through proper electrical stimulation, the neurostimulatordevices can provide pain relief for patients. The medically acceptedmechanism for pain relief is known as “gate control theory,” whichtheorizes that the nervous system has a “gate” that closes and preventsthe passage of pain signals if it is presented with sufficiently strongsensory signals. As a result, the patient may feel only a tinglysensation—also known as paresthesia—instead of pain in the area that isstimulated.

A typical neurostimulator device may include one or more integratedcircuit chips on which the control circuitry and neurostimulationcircuitry are built. The neurostimulator device may also include aplurality of electrodes that are in contact with different areas of apatient's body. Controlled by the control circuitry, the electrodes areeach capable of delivering electrical stimulation to their respectivetarget contact areas. Thus, the patient can use the neurostimulatordevice to stimulate areas in a localized manner.

Although neurostimulator devices have been proven to be useful, existingneurostimulator devices may still suffer from one or more shortcomings.For example, many existing neurostimulator devices can turn on and offeach electrode, but they lack the capability to individually control theamount of electrical stimulation given by each electrode. As anotherexample, some existing neurostimulator devices may require a largenumber of transistors to implement the neurostimulation circuitry. Thesetransistors consume a significant amount of integrated circuit chip areaand consequently drive up the fabrication costs of neurostimulatordevices.

Therefore, while existing neurostimulator devices have been generallyadequate for their intended purposes, they have not been entirelysatisfactory in every aspect.

SUMMARY

One of the broader forms of the present disclosure involves anelectrical stimulation apparatus. The apparatus includes: a powersource; a programmable switch having a first terminal and a secondterminal, wherein the first terminal is coupled to the power source; aunidirectional current source coupled to the second terminal of theswitch, the unidirectional current source having a tunable currentlevel; and a lead conductor coupled to the second terminal of the switchand the unidirectional current source, wherein the lead conductor isoperable to deliver current drawn from the unidirectional current sourcethrough an electrode contact configured for contact with a living body.

Another one of the broader forms of the present disclosure involves amedical device. The medical device contains a neurostimulator thatincludes one or more implantable channels. Each channel includes: adigitally-controlled switch coupled to a voltage source, wherein theswitch is in one of: an “on” state and an “off” state in response to afirst control signal; and a digitally-controlled current sink coupled tothe switch, wherein the current sink draws a variable amount ofelectrical current in response to a second control signal. In oneaspect, the device further includes an electrode coupled to the switchand the current sink, wherein the electrode delivers the electricalcurrent drawn by the current sink to a target tissue area.

Yet one more of the broader forms of the present disclosure involves amethod. The method includes providing a neurostimulator having differentfirst and second channels, the first channel including a first tunableunidirectional current source, the first and second channels alsoincluding: respective first and second switches each coupled to a powersupply, wherein the first current source is coupled to the power supplythrough the first switch; and respective first and second electrodescoupled to the first and second switches, respectively. The method alsoincludes entering a stimulation phase by: opening the first switch;closing the second switch; and tuning the first current source in amanner such that it sinks a programmable amount of electrical current.The method also includes entering a recovery phase by: closing both thefirst and second switches; and tuning the first current source in amanner such it does not sink any electrical current.

Another one of the broader forms of the present disclosure involves anelectrical stimulation device. The electrical stimulation deviceincludes a voltage supply means for delivering a steady voltage; aswitching means for selectively opening and closing a circuit pathcoupled to the voltage supply means; a current sink means for sinking aprogrammably-adjustable amount of current, the current sink means beingcoupled to the voltage supply means through the switching means; and aconductor means for stimulating a living body, the conductor means beingcoupled to both the switching means and the current sink means.

Yet another one of the broader forms of the present disclosure involvesan electrical stimulation device. The electrical stimulation deviceincludes an anodic channel that includes an anode electrode coupled to asteady voltage supply; and a cathodic channel that includes a currentsink that sinks a programmably-determined amount of current and acathode electrode coupled to the current sink; wherein the anodeelectrode and the cathode electrode are both implemented on a lead thatis operable to carry out electrical stimulation of a neural tissue.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isemphasized that, in accordance with the standard practice in theindustry, various features are not drawn to scale. In fact, thedimensions of the various features may be arbitrarily increased orreduced for clarity of discussion.

FIG. 1 is a simplified diagrammatic view of an embodiment of aneurostimulator device.

FIG. 2 is a simplified circuit level view of an embodiment of a channelof a neurostimulator device.

FIG. 3 is a simplified transistor level view of an embodiment of thechannel of FIG. 2.

FIGS. 4-5 are simplified circuit level views of a plurality of channelsof a neurostimulator device in a stimulation phase and a recovery phaseof an operation, respectively.

FIG. 6 is a simplified diagrammatic view of an embodiment of a paddlelead showing placements of anodes and cathodes of a neurostimulatordevice.

FIGS. 7-8 are simplified circuit level views of a plurality of channelsof a neurostimulator device in a stimulation phase and a recovery phaseof an operation, respectively, according to an alternative embodiment ofthe present disclosure.

FIG. 9 is a simplified circuit level view of another alternativeembodiment of a channel of a neurostimulator device.

FIG. 10 is a simplified transistor level view of an alternativeembodiment of the channel shown in FIG. 9.

FIGS. 11-12 are simplified circuit level views of a plurality ofchannels of a neurostimulator device in a stimulation phase and arecovery phase of an operation, respectively, according to thealternative embodiment shown in FIGS. 9-10.

FIG. 13 is a flowchart illustrating a method involving theneurostimulator device according to various aspects of the presentdisclosure.

FIGS. 14A and 14B are side and posterior views of a human spine,respectively.

DETAILED DESCRIPTION

It is to be understood that the following disclosure provides manydifferent embodiments, or examples, for implementing different featuresof the invention. Specific examples of components and arrangements aredescribed below to simplify the present disclosure. These are, ofcourse, merely examples and are not intended to be limiting. Variousfeatures may be arbitrarily drawn in different scales for simplicity andclarity.

FIG. 1 is a simplified diagrammatic view of an embodiment of aneurostimulator device 20. The neurostimulator device 20 includes anantenna 30 and a transceiver 40 coupled to the antenna 30. The antenna30 is capable of sending signals to an external device and receivingsignals from the external device. The transceiver 40 containstransmitter circuitry and receiver circuitry that together carry outdigital communication with the external device. In an embodiment, thesignals are transmitted and received at Radio Frequencies (RF).

The neurostimulator device 20 includes a microcontroller 50 that iscoupled to the transceiver 40. Based on the output of the transceiver 40(i.e., the input received from the external device), the microcontroller50 runs firmware 60, which is a control program, to operate controllogic 70. The firmware 60 includes dedicated low-level software codethat is written for a specific device, in this case the control logic70. The control logic 70 includes digital circuitry that is implementedusing a plurality of transistors, for example Field Effect Transistors(FETs). In the embodiment shown in FIG. 1, the firmware 60 and thecontrol logic 70 are integrated into the microcontroller 50. Inalternative embodiments, the firmware 60 or the control logic 70 may beimplemented separately from the microcontroller 50.

The neurostimulator device 20 includes stimulation circuitry 80 thatreceives the output of the microcontroller 50. In an embodiment, thestimulation circuitry 80 is implemented on an Application SpecificIntegrated Circuit (ASIC) chip. The stimulation circuitry 80 includeselectrical pulse generation circuitry. Based on the output of themicrocontroller 50, the electrical pulse generation circuitry generateselectrical pulses (signals) to a target tissue area. Various aspects ofthe pulse generation are described in detail in U.S. patent applicationSer. No. 13/081,896, Titled “Charge Balancing For Arbitrary WaveformGenerator & Neural Stimulation Application” and filed on Apr. 7, 2011,U.S. patent application Ser. No. 13/082,097, Titled “Arbitrary WaveformGenerator & Neural Stimulation Application With Scalable WaveformFeature” and filed on Apr. 7, 2011, and U.S. patent application Ser. No.13/081,936, Titled “Arbitrary Waveform Generator & Neural StimulationApplication” and filed on Apr. 7, 2011, each of which is herebyincorporated by reference in its entirety. Other aspects of thestimulation circuitry 80 will be discussed later in greater detail.

The neurostimulator device 20 also includes protection circuitry 90 thatis coupled to the output of the stimulation circuitry 80. In anembodiment, the protection circuitry 90 includes direct-current (DC)blocking capacitors and other electrical transient suppressioncomponents. The protection circuitry 90 protects the patient's tissuefrom unwanted electrical signals. The protection circuitry 90 alsoprotects the neurostimulator device 20 from undesirable external eventssuch as electrostatic discharge, defibrillation, or electrocautery.

The neurostimulator device 20 also includes a power source 100 and powercircuitry 110. In an embodiment, the power source 100 includes abattery. In another embodiment, the power source 100 includes a coilthat is a part of a transformer (not illustrated). In that case, thetransformer has a charging coil that is external to the neurostimulatordevice 20 and inductively coupled to the coil of the power source 100.The power source 100 therefore obtains energy from such inductivecoupling to the charging coil. In some embodiments, the power source 100may also include both a battery and a coil. The power source 100provides electrical power to the power circuitry 110. The powercircuitry 110 is coupled to the transceiver 40, the microcontroller 50,the stimulation circuitry 80. The power circuitry 110 supplies andregulates power to these coupled circuitries. In an embodiment, thepower circuitry 110 is implemented on an ASIC device.

In an embodiment, the antenna 30, the transceiver 40, themicrocontroller 50, the stimulation circuitry 80, the protectioncircuitry 90, the power source 100, and the power circuitry 110 are allcontained within a hermetically-sealed housing 150 (which may also bereferred to as a can). The housing 150 may also be considered a part ofthe neurostimulator device 20. The housing 150 may be made from titaniumor another suitable durable and/or conductive material.

A plurality of conductors (also referred to as lead wires) 170-173 runfrom the internal circuitry through hermetic feedthroughs to one or moreconnectors mounted on the hermetic enclosure. The lead wires 170-173plug into, and are removable from, those connectors. In anotherembodiment, the connectors are eliminated, and the lead wires 170-173are directly and permanently connected to the hermetic feedthroughs. Insome embodiments, the neurostimulator incorporates the electrodecontacts into its outer surface. In such embodiments, the hermeticfeedthroughs may be designed to incorporate an electrode contact in thetissue-facing side of each feedthrough, or may be designed to haveinsulated lead wires built into the neurostimulator housing, exterior tothe hermetically-sealed enclosure, that carry signals between thehermetic feedthroughs and the electrode contacts. It is understood thatthe lead wires 170-173 are shown merely as examples, and that analternative number of lead wires may be implemented, for example 16 or24 lead wires.

Electrode contacts 180-183 (also referred to as electrodes) are coupledto the lead wires 170-173. The electrode contacts 180-183 are implantedin different areas of a patient's body, where electrical stimulation isdesired. In an embodiment, an exterior portion of the housing 150 isalso used as an electrode contact. In another embodiment, one or moreelectrode contacts can be incorporated into the design of anon-conductive housing 150. In any case, the electrode contacts may alsobe considered parts of the neurostimulator system.

In an embodiment, the neurostimulator device 20 is implemented as anImplanted Pulse Generator (IPG) device, in which case all the componentsshown in FIG. 1 are surgically implanted inside the patient's body.Outside the body, the neurostimulator device 20 can be programmed usinga Clinician Programmer (not illustrated) or a Patient Programmer (notillustrated). The Clinician Programmer is used by medical personnel(such as doctors or nurses) or by others (such as sales representativesor the patient himself) to configure the neurostimulator device 20 forthe particular patient and to define the particular electricalstimulation therapy to be delivered to the target area of the patient'sbody. The Patient Programmer is used by the patient himself to controlthe operation of the neurostimulator device 20. For example, the patientcan alter one or more parameters of the electrical stimulation therapy,depending on the programming and the configuration of theneurostimulator device 20 as set by the Clinician Programmer.

In alternative embodiments, the neurostimulator device 20 can beimplemented as an External Pulse Generator (EPG). In that case, only aportion of the neurostimulator system (for example the electrodecontacts 180-183 and/or portions of the lead wires 170-173) is implantedinside the patient's body, while part or all of the neurostimulatordevice 20 remains outside the body. Other than their exact placements,the functionalities and the operations of the IPG and the EPG aresimilar. A medical device manufacturer may manufacture and provide theneurostimulator device 20 to a clinician or a patient. Clinicians mayalso provide the neurostimulator device to a patient. Some of thefunctionalities of the microcontroller 50 may be pre-programmed by themanufacturer or may be programmed by the clinician or patient.

The neurostimulator device 20 is capable of varying the amount ofelectrical stimulation delivered to each of the electrode contacts180-183. This is carried out by creating individually controllableelectrical paths, or channels. Each channel includes one of theelectrode contacts 180-183, one of the lead wires 170-173 coupled to theelectrode contact, and respective portions of the protection circuitry90 and respective portions of the stimulation circuitry 80. FIG. 2illustrates a simplified circuit diagrammatic view of an example channel200. The channel 200 includes a programmable switch 210, a current sink220 (i.e., a unidirectional current source), a protective component 230,a lead wire 240, and an electrode contact 250.

The switch 210 is powered by a voltage source HVDD, which in anembodiment is a power supply rail and is supplied by the power circuitry110 of FIG. 1. The voltage source HVDD produces a steady output voltage.The voltage source HVDD can also be programmably set to accommodate thetissue impedance of the patient. The programmability of the voltagesource HVDD helps improve power efficiency and extend battery life. Andalthough not shown in the simplified view of FIG. 2, the switch 210 iscoupled to the microcontroller 50 of FIG. 1. The microcontroller 50sends control signals to the switch 210 to either turn it on (where theswitch is closed) or shut it off (where the switch is open). In anembodiment, the switch 210 is implemented with one or more transistorsand is designed to have a low resistance when it is turned on.

The current sink 220 sinks electrical current to create an electricfield in the target tissue area of the patient's body. The electricfield generates neural signals that mask other neural signals. When theneurostimulator is used to treat pain, instead of feeling pain in thetarget tissue area, the patient feels a tingly sensation. One end of thecurrent sink 220 is coupled to the switch 210, and the other end of thecurrent sink 220 is coupled to a terminal 225. In an embodiment, theterminal 225 is tied to ground. In other embodiments, the terminal 225may be tied to another voltage level or voltage reference.

Although not shown in the simplified view of FIG. 2, the current sink220 is also coupled to the microcontroller 50 of FIG. 1. Themicrocontroller 50 sends control signals to the current sink 220 to varyits electrical current amplitude. In an embodiment, the current sink 220is implemented with a plurality of transistors. The switch 210 and thecurrent sink 220 are portions of the stimulation circuitry 80 of FIG. 1.The switch 210 and the current sink 220 will be discussed in more detaillater in association with FIG. 3.

Still referring to FIG. 2, the protective component 230 is a part of theprotection circuitry 90 of FIG. 1. As discussed above, among otherthings, the protection circuitry 90 protects the patient's tissue fromunwanted electrical signals. These unwanted electrical signals includeDC signals. If a DC component is present in the electrical stimulation(represented by a voltage or current waveform, for example) delivered,it will result in corrosion around the respective electrode contact andmay potentially harm the patient's tissue near the electrode contact.Consequently, it is desirable for the neurostimulator device 20 to onlydeliver an alternating current (AC) electrical signal to the patient.For that to happen, the neurostimulator device 20 needs to filter outany DC component in the electrical signal. Thus, in the embodiment shownin FIG. 2, the protective component 230 is implemented as a DC-blockingcapacitor. The DC-blocking capacitor has a capacitance in a range fromabout 0.05 microfarad (uF) to about 5 uF.

One end of the protective component 230 is coupled to the switch 210 andthe current sink 220. As discussed above, the switch 210, the currentsink 220, and the protective component 230 are all contained in thehermetically-sealed housing 150 of FIG. 1 according to one embodiment.The other end of the protective component 230 is coupled to the leadwire 240. In other words, the lead wire 240 extends out of the housing150, for example through a feedthrough. The lead wire 240 includes aconductive material in an embodiment. In one embodiment, the lead wiresmay include a coupling mechanism for removably receiving a connectorjoining the elongated lead wires to the IPG, the lead wires havingelectrodes attached thereto. The coupling mechanism or the connector maybe implemented inside or outside the housing 150.

The other end of the lead wire 240 is coupled to the electrode contact250. The electrode contact 250 is planted at or near the target tissuearea of the patient's body. The electrode contact 250 provideselectrical stimulation (generated by the current sink 220) to the targettissue area. It is understood that the hermetically-sealed housing 150of FIG. 1 may also serve as an electrode contact in some embodiments.Unlike the electrode contacts similar to the electrode contact 250, thehousing 150 may be driven without a current sink similar to the currentsink 220. Also, it is possible for the protective component 230 to beomitted from the housing 150.

FIG. 3 is an example transistor circuit level view of the channel 200 ofFIG. 2. The protective component 230, the lead wire 240, and theelectrode contact 250 are omitted from FIG. 3 for the sake ofsimplicity. The channel 200 includes a plurality of transistor devicesQ1-Q47. The transistor devices Q1-Q47 may each include one or moreidentical N-type FETs (NFETs) or P-type FETs (PFETs). In the embodimentshown in FIG. 3, Q1 is implemented with a PFET, and Q2 and Q30-Q47 areimplemented with NFETs. In other embodiments, Q1 may be implemented withan NFET, Q2 and Q30-Q47 may be implemented with PFETs.

Each NFET or PFET includes a gate terminal, a source terminal, a drainterminal, and a body (also referred to as bulk or substrate) terminal.Depending on the voltage levels applied to each terminal, the FET turns“on” or “off.” For ease of reference, the paragraphs below will refer tothese terminals as being terminals of the transistor devices Q1-Q2 andQ30-Q47, instead of referring to them as terminals of the FETs of thetransistor devices. Also for ease of reference, the gate terminal, thesource terminal, the drain terminal, and the body terminal may bereferred to as the gate, the source, the drain, and the body,respectively.

The letter “M” next to the transistor devices Q1-Q47 represents thenumber of identical FETs included in each transistor device. Asexamples, for the transistor device Q30, M=1, which indicates thetransistor device Q30 includes only 1 FET. For the transistor deviceQ33, M=8, which indicates the transistor device Q33 includes 8 identicalFETs. For the transistor device Q47, M=128, which indicates thetransistor device Q47 includes 128 identical FETs. These identical FETshave their respective gates coupled together; their respective drainscoupled together, their respective sources coupled together, and theirrespective bodies coupled together.

The switch 210 of FIG. 2 is implemented as the transistor device Q1. Thesource and body of the transistor device Q1 are coupled to the voltagesource HVDD. The drain of the transistor device Q1 is coupled to thedrains of the transistor devices Q30-Q37. The gate of the transistordevice Q1 is coupled to receive a binary control signal ANn through alevel shift device 300. The control signal ANn is generated by thecontrol logic 70 of the microcontroller 50 of FIG. 1. The voltage levelof the control signal ANn is within a standard voltage level range fortypical logic circuitry, which may range from 0-5 volts. The HVDDvoltage source in the embodiment shown operates at a higher voltagelevel, for example from 20 volts to 25 volts. Thus, the level shiftdevice 300 shifts the lower voltage level of the incoming control signalANn to the higher voltage level compatible with the HVDD voltage source.

In operation, the control signal ANn will turn “on” or “off” thetransistor device Q1, in effect “closing or “opening” the switch 210 ofFIG. 2. Since the transistor device Q1 serves as a switch, it isdesigned to have low impedance/resistance when it is turned on (i.e.,when the switch is closed). The impedance of the transistor device Q1needs to be much smaller than the combined impedance of the lead wire240 (FIG. 2), the electrode contact 250 (FIG. 2), and the target tissueof the patient (which can be simulated as a resistor or an RLC circuit).The dimensions of the transistor device Q1 can be tuned to ensure thatit has a low on-resistance. For example, it can be designed to have ahigh gate width to gate length ratio (W:L or W/L ratio). According toone embodiment, the on-resistance of the transistor device Q1 is in arange from about 10 ohms to about 100 ohms, for example at about 70ohms.

The transistor devices Q40-Q47 together serve as the current sink 220 ofFIG. 2. In more detail, the transistor device Q2 works in conjunctionwith the transistor devices Q40-Q47 to form a plurality of currentmirrors. An externally-supplied reference current I flows through thegate of the transistor device Q2 and establishes a voltage level at itsgate. The gates of all the transistor devices Q2 and Q40-Q47 are coupledtogether. Thus, the reference current I establishes the same voltagelevel on all the gates of the transistor devices Q2 and Q40-Q47, whichcauses the FETs of each of the transistor devices Q40-Q47 to attempt tosink the same current (the reference current I) as Q2. As discussedabove, the transistor devices Q40-Q47 include different number of FETs.Consequently, the transistor devices Q40-Q47 will attempt to sinkdifferent levels of current. For example, since the transistor deviceQ40 includes only 1 FET, it sinks the reference current I. Thetransistor device Q43 includes 8 identical FETs, so it sinks 8×I. Thetransistor device Q47 includes 128 identical FETs, so it sinks 128×I.

The gates of the transistor devices Q30-37 are coupled to an 8-bitbinary control signal (or bus) AMPn, which is also generated by thecontrol logic 70 of the microcontroller 50 of FIG. 1. In an embodiment,the transistor device Q30 is coupled to the least significant bit of thecontrol signal AMPn, and the transistor device Q37 is coupled to themost significant bit of the control signal AMPn, so on and so forth.

The drains of the transistor devices Q40-Q47 are coupled to the sourcesof the transistor devices Q30-Q37, respectively. Therefore, thetransistor devices Q30-Q37 serve as current switches that turn on or offdepending on the control signal AMPn. The transistor devices Q30-Q37 canbe individually turned on or off by the control signal AMPn. In otherwords, a subset of the transistor devices Q30-Q37 may be turned on,while a different subset of the transistor devices Q30-Q37 may be turnedoff. Since each of the transistor devices Q30-Q37 is coupled to arespective one of the transistor devices Q40-Q47, the collective currentthat is drawn by the transistor devices Q40-Q47 (i.e., the current sink220 of FIG. 2) is tunable and is controlled by the control signal AMPn.

For example, suppose the control signal AMPn has a binary value of00000111 (decimal value of 7), it will turn on the transistor devicesQ30-Q32 and turn off the transistor devices Q33-Q37. As a result,transistor devices Q40-Q42 are sinking current, while the transistordevices Q43-Q47 are not sinking current. The total amount of currentsunk by the transistor devices Q40-Q42 is (I+2×I+4×I)=7×I. As anotherexample, suppose the control signal AMPn has a binary value of 10101000(decimal value of 168), it will turn on the transistor devices Q33, Q35,and Q37 and turn off the remaining transistor devices. As a result, onlytransistor devices Q43, Q45, and Q47 are sinking current. The totalamount of current sunk by the transistor devices Q43, Q45, and Q47 is(8×I+32×I+128×I)=168×I. It can be seen now that by changing the controlsignal AMPn, the current drawn by the current sink 220 of FIG. 2 (whichis implemented with the transistor devices Q30-Q47) can be tuned to varyin amplitude anywhere from 0 (when all the transistor devices Q30-Q37are turned off) to 255×I (when all the transistor devices Q30-Q37 areturned on). The current amplitude can vary in increments of thereference current I.

It should be understood that different numbers of transistor devicesQ30-Q37 could be used, with different numbers of replications M foreach, with a corresponding number of transistor devices Q40-Q47 andcorresponding numbers of replications M for each. For example, Q30-Q37could be replaced with 255 transistor devices, each with M=1, andcorrespondingly Q40-Q47 with 255 transistor devices, also with M=1. Insuch a system, the control signal AMPn would be thermometer codedinstead of binary coded, and would be 255 bits wide. The total currentsunk by the transistor devices will be the number of “1” bits in AMPntimes I. Thus, the current amplitude can vary in increments of thereference current I. Other arrangements, including but not limited tocombinations of thermometer coding and binary coding, are also possible.

Also to ensure proper operation of the circuit, the firmware 60 orcontrol logic 70 (FIG. 1) are designed such that the control signal ANnnever turns on the transistor device Q1 when the control signal AMPn isnon-zero. Stated differently, when the current sink 220 (FIG. 2) issinking current, the switch 210 (FIG. 2) should be open. This designmakes sure the desired amount of current is pulled through the targettissue area instead of being dumped to the ground needlessly. Inaddition, the transistor devices Q30-Q37 are designed to have the samenumber of FETs as their respectively-coupled transistor devices Q40-Q47so as to improve linearity of a transfer function from the AMPn binarycode to output current.

FIG. 4 is a simplified diagrammatic view of an embodiment of a portionof the neurostimulator device 20 in a stimulation phase (or stimulationcycle) of the operation. The illustrated portion of the neurostimulatordevice 20 includes four example channels 200A-200D. The channels200A-200D include switches 210A-210D, current sinks 220A-220D(unidirectional current sources), DC-blocking capacitors 230A-230D(example protective components), and electrode contacts 250A-250D. Theelectrode contacts 250A-250D are implanted inside different target areasof a patient's tissue 320. In some embodiments, the current sinks220A-220D and the DC-blocking capacitors 230A-230D may also be implantedin or near the tissue 320. It is also understood that in someembodiments, one of the channels 200A-200D may omit the blockingcapacitor.

For the top two channels 200A-200B, their respective switches 210A-210Bare programmed to be closed, and their respective current sinks220A-220B are programmed to be drawing zero current. For the bottom twochannels 200C-200D, their respective switches 210C-210D are programmedto be open, and their respective current sinks 220C-220D are programmedto be sinking 1.2 milliamps (mA) and 1.5 mA of current, respectively. Itis understood that the numbers used here are merely examples to showthat each channel may be programmed to be sinking a different currentlevel, and that any other current level may be programmed depending onthe need of the patient. Here, the total amount of current runningthrough the tissue 320 is 2.7 mA. According to Kirchoff's current law,the sum of currents entering a node must equal to a sum of currentsleaving that node. Hence, a current I₁ flows through the channel 200A(through the capacitor 230A and the electrode contact 250A), and acurrent (2.7 mA−I₁) flows through the channel 200B.

The currents being drawn by the bottom two current sinks 220C-220Dgenerate respective electric fields near their respective electrodes250C and 250D inside the tissue 320. Depending on the current level, astronger or weaker electric field is generated, which is correlated tothe amount of sensation the patient feels with respect to the targettissue area near the electrode. The top two current sinks 220A-220B arenot sinking any current and thus do not provide any stimulation to thepatient.

In an embodiment, the channels 200A-200B not sinking current arepositioned in close proximity to the channels 200C-200D that sinkcurrent. In another embodiment, the channels 200A-200B may beimplemented in the hermetically-sealed housing 150 of FIG. 1. It is alsounderstood that although only four channels are shown in FIG. 4, theneurostimulator device 20 may contain any other number of channelssimilar to the channels 200A-200D, anyone of which is capable of sinkinga variable amount of current to stimulate a respective target area ofthe tissue 320 during the stimulation phase. According to oneembodiment, the stimulation phase lasts for about 100 microseconds toabout 150 microseconds.

The purpose of the recovery phase is to get an integral of current overthe time periods of the stimulation phase and the recovery phase tozero. If the stimulation phase is not accompanied by a recovery phase,the integral of the current (which is the amount of charge) would benon-zero, and this non-zero charge would damage the tissue 320. Therecovery phase ensures that no such net charge will be built up.Therefore the recovery phase is implemented to prevent tissue damage. Inan embodiment, the recovery phase lasts between about 4 times longerthan the stimulation phase and 10 ms. For example, the recovery phasemay last for about 400 microseconds to about 10 milliseconds (comparedto about 100 microseconds to about 150 microseconds for the stimulationphase). After the recovery phase is complete, the switches are open andthe current sinks are set to zero.

The purpose of the recovery phase is to get an integral of current overthe time periods of the stimulation phase and the recovery phase tozero. If the stimulation phase is not accompanied by a recovery phase,the integral of the current (which is the amount of charge) would benon-zero, and this non-zero charge would damage the tissue 320. Therecovery phase ensures that no such net charge will be built up.Therefore the recovery phase is implemented to prevent tissue damage. Inan embodiment, the recovery phase lasts about 4-10 times longer than thestimulation phase. For example, the recovery phase may last for about400 microseconds to about 1.5 milliseconds (compared to about 100microseconds to about 150 microseconds for the stimulation phase). Afterthe recovery phase is complete, the switches are open and the currentsinks are set to zero.

The patient is not being stimulated during the recovery phase. However,since the stimulation cycle is repeated at a frequency ranging from 15hertz (Hz) to 300 Hz, such high rate of repetition makes the stimulationfeel continuous to the patient. The patient cannot distinguish thestimulation and recovery phases based on his feelings and does not feelany interruptions in the stimulation. In other words, theneurostimulator device 20 provides constant pain relief to the patientthroughout its entire operation. It is also understood that theneurostimulator may enter the stimulation phase and the recovery phasein response to pre-set programming instructions embedded in theneurostimulator, or in response to clinician or patient control.

The embodiments of the neurostimulator device 20 discussed above offeradvantages over existing neurostimulator devices. It is understood,however, that other embodiments of the neurostimulator device 20 mayoffer different advantages, and that no particular advantage is requiredfor all embodiments. One of the advantages is reduced chip areaconsumption and therefore reduced costs. With reference to FIGS. 2 and3, the current sink 220 shown in FIG. 2 is implemented using theplurality of transistors shown in FIG. 3. For example, the transistordevice Q1 is implemented using a PFET, and the transistor devicesQ30-Q47 are implemented using 510 NFETs. In other words, implementing acurrent source similar to the current sink 220 in the transistor levelrequires a great number of PFETs (or transistors). In some existingbidirectional neurostimulator devices, each channel may include acurrent source and a current sink, where each one of them may have to beimplemented using numerous transistors. The numerous transistors consumeintegrated circuit chip area and therefore make the neurostimulatordevice more expensive.

In comparison, the neurostimulator device 20 does not need bidirectionalcurrent supplies. Each channel 200 only needs one current sink 220(unidirectional current source). The switch 210 of FIG. 2 effectivelyreplaces the current sources required for prior devices. Thus, thenumerous transistors previously needed to implement the current sourcesfor prior devices can now be replaced by a single transistor thatimplements the switch 210. In this manner, the number of transistorsrequired to implement each channel 200 of the neurostimulator device 20is almost halved, and therefore chip area consumption can be greatlyreduced. The reduction in chip area in turn leads to lower fabricationcosts.

Another advantage offered by the embodiments disclosed above is thecapability to use electrodes as “anode guards,” which is impossible inprevious neurostimulators that require bidirectional current supplies(requiring both a current source and a current sink). Anode guards areelectrode contacts that serve as anodes during the stimulation phase,and these electrode contacts also substantially encircle one or moreelectrode contacts that serve as cathodes. Such configuration helpsconcentrate the electric field (which stimulates the patient) betweenthe cathode and the encircling anodes and minimize leakage of theelectric field beyond the encircling anodes. It is understood that inother embodiments, the cathodes may be partially encircled by theanodes.

The configuration involving anodes and cathodes as discussed above isillustrated in FIG. 6 as an example, which shows a paddle-style lead 400for spinal cord stimulation. The paddle-style lead 400 is intended to beimplanted epidurally following a laminectomy. Electrode contacts 401-416are located on the paddle-style lead 400. Electrode contacts 403 and 404are configured as cathodes and are marked with “−” signs. Electrodecontacts 402, 405, 408, 409, 410, 413, 414, and 415 are configured asanodes and are marked with “+” signs. As is shown, the anode 402, 405,408, 409, 410, 413, 414, and 415 serve as anode guards and encircle thecathodes 403-404. These anodes 402, 405, 408, 409, 410, 413, 414, and415 are respectively coupled to a power supply rail (such as the voltagesource HVDD of FIG. 4) via respective switches (such as the switch 210of FIG. 4). It must be understood that some in the industry use theopposite convention for indicating cathodes and anodes, that is to say,they indicate cathodes with “+” signs and anodes with “−” signs. Theconvention of marking used is not relevant to the operation of theneurostimulator according to various aspects of the present disclosure.

The channels associated with the anodes 402, 405, 408, 409, 410, 413,414, and 415 are referred to as anodic channels, which do not sinkcurrent during the stimulation phase. Thus, the anodic channels aresimilar to the channels 200A and 200B of FIG. 4. The channels associatedwith the cathodes 403-404 are referred to as cathodic channels, which dosink current during the stimulation phase. Thus, the cathodic channelsare similar to the channels 200C and 200D of FIG. 4.

In order to minimize the leakage beyond these anodes and to concentratethe electric field between the cathodes and the anodes, it is desirablefor the anodes to all be at the same electric potential. In previousneurostimulator devices requiring bidirectional current supplies, it isextremely unlikely that the anodes will all be at the same potential.Since a bidirectional-current-supply neurostimulator controls thecurrents through each anode, the anodes will be at the same potentialonly if the current on each anode happens to have the right value tomatch the impedance between the anodes and cathodes. Since thatimpedance varies over time for various reasons, this is not likely tohappen.

Other previous neurostimulators may have only a single current sourceand can switch each electrode contact to “anode”, “cathode”, or “off”.In such neurostimulators, all of the anodes may be at the samepotential, because they are connected to the same side of the singlecurrent source. However, because that type of stimulator has only onecurrent source, it cannot control the currents through the cathodes,thereby diminishing its flexibility and usefulness.

In contrast, the neurostimulator device 20 discussed above couples allof its anode contacts to the high-voltage power rail (such as HVDD).Therefore, the anodes 402, 405, 408, 409, 410, 413, 414, and 415 areinherently all at the same potential and function well as anode guards.At the same time, the neurostimulator device 20 still has configurablecurrent sinks for every cathode, which permits the further adjustment ofthe electric field within the guard ring to target specific areas withinthe tissue 320. It is understood that FIG. 6 is only an example, andthat similar or alternative configurations are possible with otherelectrode contact configurations on a paddle lead similar to the paddlelead 400 or with other types of leads, with analogous benefitsassociated with the embodiments of the present disclosure.

The following Figures and paragraphs involve several alternativeembodiments of the neurostimulator device 20. For the sake of clarityand consistency, similar components in the following Figures will belabeled the same as they appear in FIGS. 1-6.

FIG. 7 illustrates a simplified diagrammatic view of an embodiment of aportion of the neurostimulator device 20 in the stimulation phase of theoperation. Similar to FIG. 4, FIG. 7 includes channels 200A-200D. UnlikeFIG. 5, however, all of the switches 210A-210D are open in FIG. 7.Furthermore, FIG. 7 includes an additional channel 200E. The channel200E does not include a current sink, but it does include a programmableswitch 210E that is closed, as well as a DC-blocking capacitor 230E andan electrode contact 250E that is implanted in the tissue 320.

The channel 200E may represent the above-mentioned hermetically-sealedhousing 150 of FIG. 1. Since the current sinks 220C and 220D sink 1.2 mAand 1.5 mA of current, respectively, the total amount of current flowingthrough the channel 200E and into the tissue 320 is 2.7 mA. As discussedabove, the patient only feels stimulation in areas where the channel issinking current, which in this example include channels 200C-200D. Thechannel 200E is provided to satisfy Kirchoff s current law, but it doesnot stimulate the patient. Meanwhile, the channels 200A-200B areeffectively nonexistent, since they are neither sourcing current to thechannels 200C-200D nor sinking current themselves.

FIG. 8 shows the circuit of FIG. 7 in the recovery phase. Here, theswitches 210A-210B are still open, but the switches 210C-210E areclosed. None of the current sinks 220A-220D are sinking current. Thecharges built up on the capacitors 230C-230D during the stimulationphase are discharged via the channel 200E. As discussed above, thisdischarging of the capacitors 230C-230D ensures a current integral ofabout zero over the stimulation phase and the recovery phase, therebypreventing tissue damage.

In some embodiments, the switches 210A-210B may be closed in therecovery phase. Such configuration may help remove any charge developedby leakage currents and may also help simplify the control logic.

FIG. 9 is a simplified diagrammatic view of another alternativeembodiment of a channel 430 of the neurostimulator device 20. Thechannel 430 includes similar components as the channel 200 of FIG. 2,and these similar components are labeled the same in FIG. 9 for the sakeof clarity and consistency. In addition, the channel 430 includes acharge balance switch 450 having one end coupled to the current sink 220and the protective component 230, and having the other end coupled to acommon bus 460. In an embodiment, the common bus 460 is electricallyfloating. In another embodiment, the common bus 460 is tied to a voltagereference such as the circuit ground. Though not illustrated, it isunderstood that the additional channels each include a charge balanceswitch similar to the switch 450, and all these charge balance switchesare coupled to the common bus 460. Thus, when the charge balanceswitches (including the switch 450) are closed, all the channels arecoupled to the common bus through which current can flow to equalize thevoltage across all of the capacitors (i.e., capacitors similar to theDC-blocking capacitor 230).

FIG. 10 is an example transistor circuit level view of the channel 430of FIG. 9. The protective component 230, the lead wire 240, and theelectrode contact 250 are omitted from FIG. 10 for the sake ofsimplicity. The configuration and operation of the circuit shown in FIG.10 is similar to what was shown in FIG. 3 and therefore are not repeatedfor the sake of simplicity. Unlike the circuit shown in FIG. 3, however,is that the circuit includes the charge balance switch 450 of FIG. 9,which is implemented as the transistor device Q3.

The transistor device Q3 includes an NFET whose gate is coupled to acontrol signal CSBn that is supplied by the control logic 70 of themicrocontroller 50 of FIG. 1. The control signal CSBn either turns thetransistor device Q3 on or off, thereby making it behave like aprogrammable switch. The drain of the transistor device Q3 is coupled tothe drain of the transistor device Q1 (which is the implementation ofthe switch 210 of FIG. 9). The source of the transistor device Q3 iscoupled to the common bus 460, which as discussed above may be tied tothe electrical ground, as shown in FIG. 10, or it may be electricallyfloating.

FIGS. 11-12 are simplified diagrammatic views of the alternativeembodiment including the charge balance switch during the stimulationand recovery phases, respectively. Referring to FIG. 11, theconfiguration of the circuits is similar to what is shown in FIG. 4. Inother words, the channels 430C-430D are sinking currents, while thechannels 430A-430B are working together to source the currents sunk bythe channels 430C-430D. In addition, the charge balance switches450A-450D are provided and coupled to the DC-blocking capacitors230A-230D and the common bus 460. The charge balance switches 450A-450Dare all open during the stimulation phase. The patient receivesstimulation from the channels 430C-430D, but not from channels430A-430B.

Referring now to FIG. 12, the circuit in FIG. 11 is in the recoveryphase. The current sinks 220A-220D are all programmed to be sinking nocurrent. The switches 210A-210D are all open, but the charge balanceswitches 450A-450D are all programmed to be closed. The charges storedon the DC-blocking capacitors 230C-230D are discharged through thechannels 430A-430B to ensure that the integral of current over time isstill zero. It is understood that in some embodiments, one of thechannels 430A-430D may omit the DC-blocking capacitor.

In addition to the advantages discussed above, the alternativeembodiment shown in FIGS. 9-12 offer other advantages. One advantage isincreased design flexibility. In some applications, it may be desirableto limit the peak current flow during the recovery phase. The chargebalance switches 450A-450D can be sized differently from other switches(such as switches 210A-210D) to have higher resistance than these otherswitches. The higher resistance will help limit the peak current flow.Another advantage is reduced leakage. In the real world, unwantedparasitic impedances are present in the circuits shown above (forexample, the circuits in FIGS. 2 and 9). These parasitic impedances maycause current or voltage leakages, which reduces the effectiveness ofthe neurostimulator device 20. Therefore, it may be desirable to groundthe side of the capacitor 230 coupled to the current sink 220 in orderto reduce the effects of leakage through parasitic impedances. Here, thecommon bus 460 can be tied to ground to provide the grounding of thecapacitor 230, so that the leakage effects can be alleviated.

It is also understood that in some embodiments, the neurostimulatordevice can be programmed to operate in either the mode with the chargebalance switch 450, or the mode without it. Furthermore, in anotheralternative embodiment, the neurostimulator device 20 may include thecharge balance switches 450 as well as a channel needing no currentsinks (i.e., the hermetically-sealed housing 150). In other words, theembodiment discussed above in FIGS. 9-12 may be combined with theembodiment discussed above in FIGS. 7-8.

FIG. 13 illustrates a flowchart of a method 500 involving theneurostimulator device 20. The method 500 includes block 510 in which aneurostimulator having different first and second channels is provided.The first channel includes a first tunable unidirectional currentsource. The first and second channels also include respective first andsecond switches each coupled to a power supply, wherein the firstcurrent source is coupled to the power supply through the first switch.The first and second channels also include respective first and secondelectrodes coupled to the first and second switches, respectively. Themethod 500 continues with block 520 in which the neurostimulator entersa stimulation phase by: opening the first switch; closing the secondswitch; and tuning the first current source in a manner such that itsinks a programmable amount of electrical current. The method 500continues with block 530 in which the neurostimulator enters a recoveryphase by: closing both the first and second switches; and tuning thefirst current source in a manner such it does not sink any electricalcurrent.

FIG. 14A is a side view of a spine 1000, and FIG. 14B is a posteriorview of the spine 1000. The spine 1000 includes a cervical region 1010,a thoracic region 1020, a lumbar region 1030, and a sacrococcygealregion 1040. The cervical region 1010 includes the top 7 vertebrae,which may be designated with C1-C7. The thoracic region 1020 includesthe next 12 vertebrae below the cervical region 1010, which may bedesignated with T1-T12. The lumbar region 1030 includes the final 5“true” vertebrae, which may be designated with L1-L5. The sacrococcygealregion 1040 includes 9 fused vertebrae that make up the sacrum and thecoccyx. The fused vertebrae of the sacrum may be designated with S1-S5.

Neural tissue (not illustrated for the sake of simplicity) branch offfrom the spinal cord through spaces between the vertebrae. The neuraltissue can be individually and selectively stimulated in accordance withvarious aspects of the present disclosure. For example, referring toFIG. 14B, an IPG device 1100 is implanted inside the body. The IPGdevice 1100 may include various embodiments of the neurostimulatordevice 20 described above. A conductive lead 1110 is electricallycoupled to the circuitry inside the IPG device 1100. The conductive lead1110 may be removably coupled to the IPG device 1100 through aconnector, for example. A distal end of the conductive lead 1110 isattached to one or more electrodes 1120. The electrodes 1120 areimplanted adjacent to a desired nerve tissue in the thoracic region1020. Using well-established and known techniques in the art, the distalend of the lead 1110 with its accompanying electrodes may be positionedalong or near the epidural space of the spinal cord. It is understoodthat although only one conductive lead 1110 is shown herein for the sakeof simplicity, more than one conductive lead 1110 and correspondingelectrodes 1120 may be implanted and connected to the IPG device 1100.

The electrodes 1120 deliver current drawn from the current sources inthe IPG device 1100, therefore generating an electric field near theneural tissue. The electric field stimulates the neural tissue toaccomplish its intended functions. For example, the neural stimulationmay alleviate pain in an embodiment. In other embodiments, a stimulatoras described above may be placed in different locations throughout thebody and may be programmed to address a variety of problems, includingfor example but without limitation; prevention or reduction of epilepticseizures, weight control or regulation of heart beats.

It is understood that the IPG device 1100, the lead 1110, and theelectrodes 1120 may be implanted completely inside the body, may bepositioned completely outside the body or may have only one or morecomponents implanted within the body while other components remainoutside the body. When they are implanted inside the body, the implantlocation may be adjusted (e.g., anywhere along the spine 1000) todeliver the intended therapeutic effects of spinal cord electricalstimulation in a desired region of the spine. Furthermore, it isunderstood that the IPG device 1100 may be controlled by a patientprogrammer or a clinician programmer 1200.

The IPD device 1100 may be set in a trialing mode to test differentgroups of stimulation patterns. For example, in an embodiment, apaddle-style lead (such as the one shown in FIG. 6) is used as a lead tostimulate the neural tissue. The paddle-style lead has a plurality ofelectrodes that can each be programmably set as a cathode or an anode.The patient or the clinician may programmably enter a first stimulationpattern, in which a subset of electrodes on the lead are designated ascathodes, and a different subset of electrodes on the lead aredesignated as anodes. In accordance with the discussions associated withFIG. 6 above, the group of anodes may at least partially encircle thecathodes so as to minimize leakage of the electric field beyond theencircling anodes. The current flow between the anodes and the cathodesserves to electrically stimulate a desired adjacent area of neuraltissue by generating an electric field in the tissue. The selection ofthe cathode and anode subset may be made to minimize electric fieldleakage and thus reduce stimulation of undesired areas of neural tissue.

These anodes on the lead all receive a steady voltage delivered by thevoltage supply (e.g., HVDD) and may all have substantially identicalvoltage potentials. The cathodes on the lead are coupled to theirrespective current sinks and therefore may have different potentialsthan the anodes or from one another. The cathodes serve to electricallystimulate a desired adjacent area of neural tissue by generatingelectric fields through the current drawn from the current sinks.Meanwhile, the anodes are serving as “anode guards” to minimize electricfield leakage.

The patient may then decide to try a second stimulation pattern that isdifferent from the first stimulation pattern. The second stimulationpattern sets different groups of electrodes as anodes and cathodes thanthe first stimulation pattern. The cathodes of the second stimulationpattern will carry out electrical stimulation of a different area of theneural tissue, while the anodes still serve as anode guards for thesecathodes. The patient may decide whether the first stimulation patternor the second stimulation pattern is better. He may also try anadditional number of different stimulation patterns until he finds theone he prefers.

The foregoing has outlined features of several embodiments so that thoseskilled in the art may better understand the detailed description thatfollows. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions andalterations herein without departing from the spirit and scope of thepresent disclosure.

1. An electrical stimulation apparatus, comprising: a power source; aprogrammable switch having a first terminal and a second terminal,wherein the first terminal is coupled to the power source; aunidirectional current source coupled to the second terminal of theswitch, the unidirectional current source having a tunable currentlevel; and a lead conductor coupled to the second terminal of the switchand the unidirectional current source, wherein the lead conductor isoperable to deliver current drawn from the unidirectional current sourcethrough an electrode contact configured for contact with a living body.2. The electrical stimulation apparatus of claim 1, wherein the powersource includes a voltage source; and wherein the electrode contact isimplantable in the human body tissue and is detachably coupled to thelead conductor.
 3. The electrical stimulation apparatus of claim 1,further including a protective component that is coupled between theelectrode contact and the unidirectional current source.
 4. Theelectrical stimulation apparatus of claim 3, wherein the protectivecomponent includes a direct current (DC) blocking capacitor.
 5. Theelectrical stimulation apparatus of claim 1, wherein the apparatus is aneurostimulator device, the neurostimulator device further including amicrocontroller that controls opening and closing of the switch and setsthe current level of the unidirectional current source.
 6. Theelectrical stimulation apparatus of claim 5, wherein the switch, theunidirectional current source, and the lead conductor are portions of achannel of the neurostimulator device; and wherein the neurostimulatordevice includes a plurality of additional channels that can each delivera variable amount of electrical stimulation to a different area of thebody tissue in which the electrode contacts of the respective channelsare implanted.
 7. The electrical stimulation apparatus of claim 1,further including a charge balance switch coupled between theunidirectional current source and a common bus.
 8. The electricalstimulation apparatus of claim 1, wherein the unidirectional currentsource includes a plurality of current mirrors that can each sink adifferent amount of current.
 9. The electrical stimulation apparatus ofclaim 8, wherein the unidirectional current source further includes aplurality of current switches that are each coupled to a respective oneof the current mirrors, and wherein in response to a control signal, thecurrent switches can individually turn on to allow the respectivecurrent mirror to sink current, or turn off to prevent the respectivecurrent mirror from sinking current.
 10. A medical device, comprising: aneurostimulator that includes one or more channels, wherein each channelincludes: a digitally-controlled switch coupled to a voltage source,wherein the switch is in one of: an “on” state and an “off” state inresponse to a first control signal; a digitally-controlled current sinkcoupled to the switch in a manner such that the switch is coupledbetween the current sink and the voltage source, wherein the currentsink draws a variable amount of electrical current in response to asecond control signal; and a conductor coupled to the switch and thecurrent sink, wherein the conductor is configured to be coupled to anelectrode that is operable to deliver the electrical current drawn bythe current sink to a target tissue area.
 11. The medical device ofclaim 10, wherein each channel further includes a capacitor that isoperable to filter out a direct current (DC) component in the electricalcurrent delivered by the electrode.
 12. The medical device of claim 10,wherein the neurostimulator further includes: power circuitry thatsupplies the voltage source; transceiver circuitry that facilitatescommunication with one or more external devices; and control circuitrythat generates the first and second control signals in response to thecommunication with the one or more external devices.
 13. The medicaldevice of claim 12, wherein the control circuitry is operable togenerate and send different versions of the first and second controlsignals to each of the channels in a manner so that a subset of thechannels are turned on while a different subset of the channels areturned off.
 14. The medical device of claim 12, wherein the controlcircuitry generates the first and second control signals in a mannersuch that the switch is never turned on when the current sink is drawingcurrent.
 15. The medical device of claim 10, further including a chargebalance switch coupled between the current sink and a common bus, thecommon bus being tied to one of: a floating node, a voltage reference,and an electrical ground.
 16. The medical device of claim 10, wherein:the second control signal includes a plurality of bits; the current sinkincludes a plurality of first transistors and a plurality of secondtransistors; the first transistors are each coupled to the switchthrough a respective one of the second transistors; the firsttransistors are each operable to draw a current that is an integermultiple of a reference current; and the second transistors are operableto be individually turned on or off by a corresponding bit of the secondcontrol signal, thereby enabling the respectively-coupled firsttransistor to draw current or preventing such first transistor fromdrawing current.
 17. A method, comprising: providing a neurostimulatorhaving different first and second channels, the first channel includinga first tunable unidirectional current source, the first and secondchannels also including: respective first and second switches eachcoupled to a power supply, wherein the first current source is coupledto the power supply through the first switch; and respective first andsecond electrodes coupled to the first and second switches,respectively; entering a stimulation phase by: opening the first switch;closing the second switch; and tuning the first current source in amanner such that it sinks a programmable amount of electrical current;and entering a recovery phase by: closing both the first and secondswitches; and tuning the first current source in a manner such that itdoes not sink any electrical current.
 18. The method of claim 17,wherein the providing the neurostimulator is carried out in a mannersuch that the second channel further includes a second tunableunidirectional current source that is operable to sink a differentamount of electrical current than the first current source; wherein theentering the stimulation phase further includes tuning the secondcurrent source in a manner such that it sinks no electrical currentduring the stimulation phase; and wherein the entering the recoveryphase further includes tuning the second current source in a manner suchthat it sinks no electrical current during the recovery phase.
 19. Themethod of claim 17, wherein the providing the neurostimulator is carriedout in a manner such that the first channel includes a direct current(DC) blocking capacitor; and wherein the recovery phase is carried outin a manner such that the DC blocking capacitor discharges during therecovery phase.
 20. The method of claim 17, wherein the enter thestimulation phase and the entering the recovery phase are carried out ina manner such that an operating period for the recovery phase is atleast multiple times longer than an operating period for the stimulationphase.
 21. An electrical stimulation device, comprising: a voltagesupply means for delivering a steady voltage; a switching means forselectively opening and closing a circuit path coupled to the voltagesupply means; a current sink means for sinking a programmably-adjustableamount of current, the current sink means being coupled to the voltagesupply means through the switching means; and a conductor means forstimulating a living body, the conductor means being coupled to both theswitching means and the current sink means.
 22. The electricalstimulation device of claim 21, further including an electrode means forcarrying out the stimulating of the living body by delivering currentdrawn from the current sink means to a designated region of the body,the electrode means being coupled to the conductor means.
 23. Theelectrical stimulation device of claim 22, further including aprotective means for removing a direct current (DC) component from thecurrent delivered to the body, the protective means being coupledbetween the electrode means and the current sink means.
 24. Theelectrical stimulation device of claim 21, further including amicrocontroller means for: putting the switching means in one of: an“open” state and a “closed” state; and setting the amount of currentsunk by the current sink means.
 25. The electrical stimulation device ofclaim 24, further including a plurality of additional current sink meansbeing coupled to the voltage supply means through a respectiveadditional switching means; wherein the microcontroller means isoperable to: put a subset of the switching means to the “open” statewhile putting a different subset of the switching means to the “closed”state; and set a subset of the current sink means to sink a respectiveamount of current while setting a different subset of the current sinkmeans to sink no current.
 26. The electrical stimulation device of claim21, wherein the current sink means includes: a plurality of currentmirror means for sinking currents that are different integer multiplesof a reference current; and a plurality of current mirror switch meansfor selectively allowing a subset of the current mirror means to sinkcurrent while preventing a different subset of the current mirror meansfrom sinking current; wherein each of the current mirror means iscoupled to a respective one of the current mirror switch means.
 27. Anelectrical stimulation device, comprising: an anodic channel thatincludes an anode electrode coupled to a steady voltage supply; and acathodic channel that includes a current sink that sinks aprogrammably-determined amount of current and a cathode electrodecoupled to the current sink; wherein the anode electrode and the cathodeelectrode are both implemented on a lead that is operable to carry outelectrical stimulation of a neural tissue.
 28. The electricalstimulation device of claim 27, further including a plurality ofadditional anodic channels; wherein the anode electrodes of all theanodic channels are positioned on the lead in a manner such that thecathode electrode is at least partially encircled by the anodeelectrodes.
 29. The electrical stimulation device of claim 28, whereinall the anode electrodes have identical voltage potentials.
 30. Theelectrical stimulation device of claim 27, wherein the lead includes apaddle-style lead.
 31. The electrical stimulation device of claim 27,wherein: the anodic channel and the cathodic channel each include aprogrammable switch through which the anode electrode and the cathodeelectrode are each coupled to the voltage supply; the switch of theanodic channel is programmed to be in a closed position; and the switchof the cathodic channel is programmed to be in an open position.
 32. Theelectrical stimulation device of claim 31, wherein: the current sink ofthe cathodic channel is coupled to the voltage supply through theopen-position switch of the cathodic channel; the anodic channelincludes a current sink that is coupled to the voltage supply throughthe closed-position switch of the anodic channel; and the current sinkof the anodic channel sinks no current while the current sink of thecathodic channel sinks a non-zero amount of current.
 33. The electricalstimulation device of claim 27, further including a microcontroller thatis operable to: program the amount of current sunk by the current sinkof the cathodic channel; set the switch of the anodic channel to be inthe closed position; and set the switch of the cathodic channel to be inthe open position.
 34. The electrical stimulation device of claim 27,wherein: the anodic channel includes a direct-current-blocking capacitorthat is coupled between the anode electrode and the voltage supply; andthe cathodic channel includes a direct-current-blocking capacitor thatis coupled between the cathode electrode and the current sink.